This invention relates to nuclear power plant steam generators and is more particularly directed to methods and devices for removing radioactive contaminants from the internal surfaces of the primary fluid inlet and outlet headers, including the divider plate (if so fitted), the tube sheet surface exposed to the primary fluid, and portions of the primary fluid side of the tubes.
Steam generators for nuclear service are typically of either a U-tube or once-through configuration. While this invention is applicable to both, for purposes of describing this invention the U-tube type steam generator will be considered.
A typical U-tube type nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, the tubes having two straight sections joined at their upper end by a pipe bend, a tube sheet for supporting the tubes at the ends of the tube straight section, a dividing plate that cooperates with the tube sheet forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet header, and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet header. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacent the shell, and a feedwater inlet system above the pipe-bend end of the tube bundle. The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet nozzle. From there, the primary fluid is conducted into the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator through the feedwater ring. The feedwater is conducted down the annular chamber adjacent the shell until the tube sheet near the bottom of the annular chamber causes the feedwater to reverse direction, and pass in heat-transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the feedwater is circulating in heat-transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feedwater surrounding the tubes, causing a portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical steam turbine electrical generating equipment to produce electricity.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tube walls, the latter serving as primary boundary for isolating these radioactive particles, it is important that the U-tubes be maintained defect-free and that no breaks occur in the U-tubes. However, experience has shown that under certain conditions the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater. This can present a highly undesirable and potentially dangerous condition.
Testing or inspection is required at regular intervals to determine the condition of the tubes. Such testing conducted according to standard techniques requires personnel to enter the inlet and outlet headers through the manways provided for that purpose. Deposits of radioactive particles on primary fluid wetted surfaces result in significant personnel radiation exposure rates in areas where personnel access is required. This limits the amount of time that personnel can remain in the headers, and restricts the amount of testing that each individual worker can perform.
A reduction of this radiation dose rate to some practical limit is sometimes attempted prior to testing, inspections or other work being carried out in the inlet and outlet headers.
One known method for removal of a portion of these deposits of radioactive particles on the internal surfaces of the inlet and outlet header involves impinging a high velocity stream of water against these surfaces. This cleaning process (commonly referred to as decontamination) is also known as hydroblasting, hydrolancing, or high-pressure spraying. A decontamination factor (i.e., exposure rate before cleaning divided by exposure rate after cleaning) of two can typically be expected in the header following decontamination by this method. The several shortcomings inherent in the high pressure spraying process include the relatively low decontamination factor and the high radiation exposures received by personnel involved in carrying out the cleaning process.